Backup power supply.

As far as i know you can't, though it is an interesting idea.

Unless you're an electrical engineer, I doubt you're going to be able to do that. Plus, from my limited knowledge of circuits (I'm a Nuke student), it wouldn't even work that way because if one failed, the other one wouldn't work. They'd have to be hooked up in parallel and both plugged into the motherboard at the same time.

You could always buy another PSU and keep it in the closet after you test to make sure it works. Just keep it in an anti static bag so dust doesn't build up inside it or anything.

I think some server boards have the ability to use two PSUs at once. But as boiler mentioned unless your board has two main ATX plugs its not going to work.

That's an interesting thought on your part. It would be a very good thing to give it a try and see if it works out in a proper manner. Giving it a second thought, I would like to add that though it sounds quite simple but to have it implemented in a proper manner would be a tedious job. The first things is that it has to be parallel and so any problem in one will in one way or the other affect the other too.

You can, its called "Redudant PSUs", its a pretty common thing for servers.